Workers in an oil and gas refinery plant are exposed to the danger of touch voltage that occurs due to lightning strike. Horizontal grounding grid with minimum mesh size is the best precaution to eliminate this risk. However, this prevention method requires a lot of money. Henceforth, a series of calculations and simulations have been conveyed in order to obtain the most advantageous and proper grounding grid mesh size. Simulations were carried out by using CDEGS software. In this study, the mesh sizes were varied from 6.5m x 6.5m to 20m x 20m. The simulation results indicate that the peak value of maximum transient touch voltage rose for when the mesh size increases. Moreover, when the mesh size smaller, the injected current dispersed and flowed towards the earth faster, making the safety perimeter radius shorter. Nevertheless, grounding grid with smaller mesh size had more total conductor length. As a result, the cost would also increase. The results conclusively show that 10m x 10m is the most optimum grounding grid mesh size.